But the legitimacy of this theory cannot be admitted, and the calorific movement should also be a phenomenon so strictly confined in space that our most delicate means of investigation would not enable us to perceive it.

It is better, then, to continue to regard the unit of difference of temperature as a distinct unit, to be added to the fundamental units.
To define the measure of a certain temperature, we take, in practice, some arbitrary property of a body.

The only necessary condition of this property is, that it should constantly vary in the same direction when the temperature rises, and that it should possess, at any temperature, a well-marked value.

We measure this value by melting ice and by the vapour of boiling water under normal pressure, and the successive hundredths of its variation, beginning with the melting ice, defines the percentage.

Thermodynamics, however, has made it plain that we can set up a thermometric scale without relying upon any determined property of a real body.